1. Field of the Invention
The present invention relates to the manufacturing of semiconductor components and more particularly of power semiconductor components having deep grooves such as lateral grooves for interrupting a junction under the surface of the semiconductor component to form a so-called "mesa" structure.
2. Discussion of the Related Art
As used herein, "deep groove" is to be construed as a groove with a depth within the range 50-150 .mu.m.
First, conventional groove etching techniques will be described with reference to FIGS. 1A-1C and 2A-2C, to point out the drawbacks of the prior art and the problems encountered therewith.
The conventional technique illustrated in FIGS. 1A-1C simply uses a photoresist mask 1 formed on a silicon substrate 2. The mask 1 comprises a window 3 serving to delineate an area for subsequent forming of a groove. Then, the wafer is immersed into a photoresist-selective bath for etching the silicon substrate 2 in the area delineated to form groove 4.
This very simple method is suitable only for etching shallow grooves, shallower than 50 .mu.m in depth, because of the following drawbacks:
first, in the etching bath, either the bath is stirred or the substrate immersed therein is vibrated to ensure a satisfactory homogeneity of the etchant. When the substrate is vibrated, once etching has started, portions 5 (FIG. 1B) of the photoresist mask 1 are vibrated too in the directions shown by arrows 7, which photoresist mask vibration causes the photoresist mask 1 to become dislodged (unstuck) from the silicon substrate 2 upon which the mask 1 is disposed;
owing to the dislodging of the photoresist mask 1 resulting from the above operation, some etchant penetrates the substrate 2 at the interface between the photoresist mask 1 and the silicon substrate 2, causing localized overetching of the silicon under the mask, as indicated by reference character 6 in the cross-sectional view of FIG. 1B and in the top down view of FIG. 1C. The resulting poor definition of the groove edges degrades the electric properties of the resulting component;
once a groove has been formed, it is often desired, after rinsing and testing of the component, to perform an additional etching operation, so-called "reviving"; when using a photoresist mask, it is virtually impossible to perform such an additional etching operation because, after the rinse phase, the mask is further dislodged from the substrate and the etchant, therefore, would penetrate the substrate at the interface between the mask and the substrate, which etchant penetration would further degrade the component performance; and
when a photoresist mask is used, it has been observed that the lateral extension of etching below the mask is substantially equal to the depth of the vertical etching. In other words, the ratio between the lateral and vertical etchings is approximately equal to 1:1.
In order to improve results obtained in the case of deep groove etchings, the conventional method illustrated in FIGS. 2A-2C has been used. With this method, the photoresist layer 1 is deposited on a silicon oxide layer 10. The silicon oxide layer 10 is etched away beneath the previously formed window 3 in the photoresist layer 1 (the mask) to form a groove 4.
This method has the following advantages over the above process. The presence of the silicon oxide layer 10, relatively rigid beneath the photoresist layer 1, prevents the overlying photoresist and oxide portions from vibrating and therefore reduces the problems caused by the dislodging of the photoresist layer. In addition, when etching is performed beneath an oxide layer, the ratio between the lateral and vertical etchings is approximately 0.75 instead of 1. The groove is, therefore, more accurately defined. Further, etching can be prolonged to provide grooves with a depth up to a few hundred yam (for example, 100-200 .mu.m). However, this method brings about the following other drawbacks:
if the silicon oxide is relatively rigid, it is brittle and portions of the overlying silicon oxide layer may crack during the etching operation and may deposit into the groove, which will cause localized etching defects;
at the end of the etching operation, the photoresist layer is removed with a selective etchant. Then, the overlying oxide portions are conventionally broken with pressured water jets. However, some oxide caps 13, 14 (FIGS. 2B and 2C) may remain after the breaking operation;
if a reviving operation, which includes an additional etching of the silicon groove at the end of the process, is performed, a new silicon oxide cap results. The drawback of this cap (as illustrated in FIG. 2B) includes that, when a passivation product, for example glass 11, is deposited in the groove, the thickness of the glass will be reduced at the vicinity of the cap, which decreases the protection quality thereof and generates areas where electric arcs are liable to occur.
Furthermore, those skilled in the art will appreciate that, with this method, at the end of the groove etching operation, the adherence between the photoresist and the silicon oxide is not quite satisfactory, since the photoresist has been subjected to several successive aggressions, namely, the aggression resulting from the etching of the oxide layer, the aggression corresponding to the etching of the groove and to intermediate and final rinsing operations. Thus, it is hazardous with this method to etch the silicon oxide layer with a selective etchant prior to eliminating the photoresist layer because, in such a case, the etchant of the silicon oxide is at risk to deeply penetrate between the photoresist layer and silicon oxide layer, which is liable to reduce, at least locally, the thickness of the silicon oxide and to pollute sensitive areas spaced apart from the groove.
FIG. 2C shows a top down view of the groove obtained with the method described above and shown in FIGS. 2A and 2B. The groove edges are linear, as indicated by line 13, but there remain irregular protruding oxide caps 14 that may lead to deterioration of the electric properties of the component, as disclosed above.